1. Field of the Invention
This invention relates to an image forming apparatus making use of an amorphous silicon photosensitive member.
2. Related Background Art
In recent years, in full-color copying machines or full-color printers, it is required to enrich their functions as black-and-white machines. That is, even in full-color copying machines or full-color printers, the same speed and high image quality as those in black-and-white machines are required in monochromatic image formation, and in addition thereto there is an increasing need for copying machines or printers that can form full-color images with sharpness and high image quality. In such copying machines or printers, they are frequently used for black color alone, and toners therefor are also necessarily used in a large quantity. Accordingly, black toners in future full-color copying machines or printers are required to have higher image reproduction performance and running stability.
In electrophotography, many developing methods are known in the art. In particular, developing methods making use of magnetic developers are superior in view of running stability and running cost.
Meanwhile, in high-speed machines required to have running stability and high reliability, amorphous silicon photosensitive members are preferably used. The amorphous silicon photosensitive members have advantages that they have high sensitivity over the whole region of visible light and have so high surface hardness as to have superior durability, heat resistance and environmental stability.
Usually, toner particles having participated in development on a photosensitive member in an electrophotographic process are transferred to a transfer material such as paper, where residual toner particles not transferred thereto and having remained on the photosensitive member are removed by a cleaning member. It, however, is difficult to remove the residual toner particles completely therefrom, and residual toner particles having not been removed remain on the photosensitive member surface to become adhere or cling to the surface of the photosensitive member unwantedly. The toner particles remaining on the surface of the photosensitive member and the toner particles having adhered or clung to the surface of the photosensitive member usually do not come into question because they are usually scraped off together with the surface of the photosensitive member by the friction with toner particles or other members in the subsequent developing step or transfer step.
However, the amorphous silicon photosensitive members have so high hardness that their surfaces can not easily be scraped, and it is difficult to remove the toner particles having adhered or clung to the surface of the photosensitive member.
In addition, digital copying machines chiefly employ a method in which electrostatic latent images are formed by means of a laser. Hence, in order to achieve a high-resolution and high-minuteness developing system, it has been put forward to make toner particles have small particle diameter. However, such toner particles made to have small particle diameter are not well removable by cleaning, and hence it is attempted to improve cleaning performance by, e.g., making contact pressure of a cleaning blade higher against the photosensitive member. However, where magnetic toner particles are used, the magnetic material having come bare to toner particle surfaces may scratch the surface of the photosensitive member to cause deterioration of image quality.
Toner particles are also usually present at the part where the cleaning blade and the photosensitive member come into contact with each other. Such toner particles present there a little have the function of lubrication between the cleaning blade and the photosensitive member to make always good cleaning performable. However, it is known that, if such toner particles reduce abruptly, the lubricity may become locally poor, so that the cleaning blade may turn up in the rotational direction of the photosensitive member or may vibrate on the photosensitive member to come into a state that it can not remove the residual toner particles on the photosensitive member. Such a problem is more remarkable as higher speed is achieved (process speed is higher).
In order to keep such a phenomenon from occurring, it is practiced that, for the purpose of stable feed of toner particles to the cleaning blade, a magnet roller is placed on the upstream side of the cleaning blade in the rotational direction of the photosensitive member to apply toner particles on the photosensitive member at the same time the residue matter on the photosensitive member is scraped off by rubbing. By such a means, toner particles collected by cleaning do form a magnetic brush, which feeds the toner particles again to the surface of the photosensitive member. This has brought certain effect in respect of cleaning performance of systems making use of magnetic developers.
Aiming at further improvement of these, it is proposed to incorporate an inorganic fine powder as an abrasive or a lubricant in a magnetic developer. For example, it is disclosed that conductive zinc oxide and tin oxide are incorporated (e.g., Japanese Patent Applications Laid-open No. S58-66951, No. S59-168458, No. S59-168459, No. S59-168460 and No. S59-170847) or that cerium fluoride or fluorine-containing cerium oxide particles are incorporated (e.g., Japanese Patent Applications Laid-open No. H1-204068 and No. H8-82949). In these methods, however, any stable image density is not obtainable when digital high-speed development is performed, or abrasive particles may non-uniformly scrape the photosensitive member because their hardness is not uniform. This has tended to make the blade turn up and to cause slip-through of toner particles, because of a difference in coefficient of friction between the photosensitive member and the cleaning blade at the abraded part and the unabraded part.
Non-magnetic toners as commonly used as color toners. In the case when as stated above the magnetic developer is used as a black developer of a full-color copying machine, it is difficult to well remove both the non-magnetic toner and the magnetic-developer by cleaning, because of a difference in proper cleaning conditions between the both. It is also prevalent to use a magnetic-brush cleaning member as a cleaning auxiliary member, which, however, tends to lower the cleaning performance on the surface of the photosensitive member when full-color copying is relatively frequently used. This phenomenon may be remarkable especially when polymerization toners are used as non-magnetic color toners for the purpose of improving transfer efficiency and so forth. Toners produced by polymerization commonly have a high circularity, and hence the toners may frequently slip through the cleaning blade and this may further make poor the lubricity between the blade and the photosensitive member, so that a local force may be applied to the blade to cause its edge to chip.
Meanwhile, it is attempted to reproduce fine lines sharply even without making average particle diameter small so much, using a magnetic toner to which coarse particles are added to make the toner contain 16 μm or larger particles in a proportion of 2.1 to 4.0% by volume to broaden particle size distribution (Japanese Patent Application Laid-open No. 2001-249488). Also, as a technique concerning the mixture of a plurality of magnetic toners, there is a disclosure of a magnetic toner having two peaks in particle size distribution, in the region of 50 μm or less (Japanese Patent Application Laid-open No. S56-29248). In these magnetic toners, the toners have a relatively good charging performance at the beginning of use, but may come to have an unstable charging performance with progress of running over a long period of time while repeating replenishment. Especially in severe environment such as a high-temperature and high-humidity environment, image density may decrease, or the toner tends to participate in development at non-image areas to cause fog.
It is also attempted to improve transfer performance and so forth by incorporating coarse particles in an appropriate quantity (e.g., Japanese Patent Applications Laid-open No. 2002-91053, No. 2000-10334, No. 2002-49172 and No. 2002-162772). If, however, it is attempted to use such a toner in magnetic toners, the charging stability, running stability and so forth just owing to the magnetic toners may be damaged because the size of the coarse particles is unsuitable or because their quantity is too large.
In addition, in the above techniques, coarse particles which are so large as to be more than 100 μm are contained in a relatively large quantity in many cases. The presence of such coarse particles at the cleaning blade tends to make white lines appear on images or cause the blade edge to chip to cause image defects. Furthermore, there has been a problem such that, when used in, e.g., a full-color copying machine making use of a magnetic black toner and non-magnetic color toners in combination, faulty cleaning may occur for the reasons stated above.
Thus, in magnetic developers used in the one-component developing system, any magnetic developer has not been materialized that has running stability and charging stability well fittable to high-speed digital machines and at the same time can exhibit superior cleaning performance even when used in combination with non-magnetic color toners.